1. Field of the Invention
This invention is adapted especially for hardfacing on tool joints used with drill pipe for earth boring operations, particularly those used inside casing which may be damaged due to excessive wear from some of the more conventional tool joint hardfacings.
2. Description of the Prior Art
The most common drill pipe used in earth boring operations has connection members or tool joints on each end that are larger in diameter than the drill pipe. Annular bands of hardfacing are commonly deposited on each tool joint. One type hardfacing has macroscopic sintered tungsten carbide granules within and alloy steel matrix. Sintered tungsten carbide granules, as explained in U.S. Pat. No. 3,800,891, comprise microscopic grains of tungsten carbide held together by a binder of an iron group metal, usually cobalt. Sintered tungsten carbide hardfacing is normally applied on tool joints by rotating the tool joint, providing an arc with a consummable steel wire, discharging an inert gas around the wire, and gravity feeding sintered tungsten carbide particles into the weld puddle behind the wire.
One disadvantage of the resulting sintered tungsten carbide hardfacing is that many of the granules remain only partially embedded in the matrix, giving a rough abrasive exterior. In deep wells, intermediate strings of casing are set as the well is drilled. While drilling deeper through a string of intermediate casing, the rough surface of the hardfacing can abrade and damage the casing. Consequently, it is advantageous to have a hardfacing surface free of protruding tungsten carbide granules. Pure alloy hardfacings have not been found as wear resistant as tungsten carbide granule hardfacing. One prior art hardfacing employs one layer of an alloy surface layer applied over a first layer of tungsten carbide granule hardfacing. This may be satisfactory when properly applied but adds an additional operation since two layers are used. Further, dual layers of hardfacing may tend to crack more due to the thermal shock of reheating the first layer. Also, it can result in poor granule distribution if reheating is not accurately controlled.
Another prior art hardfacing employed cast tungsten carbide particles of approximately 100 mesh size, which is much smaller than the preferred sintered tungsten carbide granules. The smallest sintered tungsten carbide granules now in common usage are approximately 45 mesh. Cast tungsten carbide, as explained in U.S. Pat. No. 3,800,891, is essentially an eutectic of monotungsten carbide and ditungsten carbide, with no additional material holding the grains of a particle together. Such granules when dropped directly into the arc tend to bury deeply in the molten matrix. The resulting hardfacing was not as wear resistant as hardfacings containing large size cast tungsten carbide particles, although the surface was smoother.
Feeding sintered tungsten carbide granules directly into the arc was thought to be undesirable, even though in the past the smaller size cast tungsten carbide particles were fed directly into the arc. Cast tungsten carbide melts at a much higher temperature than sintered tungsten carbide, which was expected to dissolve excessively if fed directly into the arc.